1. FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device and to a method for driving a display device. In particular, the present invention relates to a driving circuit for generating a driving waveform that provides uniform display quality and peripheral circuitry thereof.
2. DESCRIPTION OF THE RELATED ART
In recent years, there has been increasing demand for display devices capable of displaying a large amount of information at the same time due to the rise of a highly information-oriented society. CRT (Cathode Ray Tubes) displays have conventionally been used for such purposes. However, CRTs are generally large and tend to consume a large amount of power, making them unsuitable for use other than as desk-top devices. On the other hand, flat display devices such as LC (liquid crystal) display devices are attracting much attention because of their thinness and light weight.
LC display devices were originally developed as display devices for calculators, watches, etc. However, current LC display devices typically include a matrix of scanning electrodes and data electrodes, and are capable of displaying images on a large screen owing to progress in technology concerning STN (Super-Twisted Nematic) liquid crystal and TFT (Thin Film Transistor) elements.
Such matrix type LC display devices can be classified into simple matrix type display devices and active matrix type display devices in terms of their driving methods.
Active matrix type LC display devices, which are typically driven by using TFT elements or MIM (Metal Insulator Metal) elements, include a matrix of scanning electrodes and data electrodes with switching elements of TFTs, diodes, and the like located at the respective intersections of the scanning and data electrodes. A display is realized by controlling such switching elements so as to apply a voltage independently to portions of liquid crystal corresponding to the respective pixels. In such active matrix type LC display devices, the LC is usually driven in its TN (Twisted Nematic) mode, thereby achieving high contrast and a quick response at the same time. Since the voltage to be applied to each portion of LC corresponding to a pixel can be independently controlled, it is relatively easy to display intermediate gray scale tones.
On the other hand, a typical simple matrix type LC display device driven in a STN mode includes an LC layer interposed between glass substrates having a matrix of electrodes formed on the surface thereof so as to conduct display by utilizing the steep characteristics of the electrooptical effects of LC, i.e., the change in the optical characteristics of LC when an electric field is applied thereto. As a result, simple matrix type LC display devices require a relatively simple panel structure and production process, and therefore are more preferable in terms of cost than active matrix type LC display devices.
Simple matrix type STN LC display panels have conventionally been driven by a time-divided method (or "duty driving") which is also referred to as a linearly sequential driving method. Since a plurality of pixels are coupled to one electrode in an active matrix type LC display device, the applied voltage has time-divided pulses. Generally, scanning electrodes are linearly sequentially scanned at a frame cycle of 20 ms or less. A large selection pulse is applied to each scanning electrode once per frame, in synchronization with which a data signal is applied via a data electrode.
Since conventional STN LC display devices have a relatively low LC response speed, e.g., 300 ms, the LC can respond in accordance with the ON/OFF ratio of the effective voltage applied in linearly sequential driving, thereby achieving a practical contrast level. However, once quick response is realized in STN LC display devices (such that moving images can be displayed thereby) by reducing the viscosity of LC and/or reducing the thickness of the LC layer, etc., the linearly sequential driving results in a noticeable degradation of contrast due to a so-called frame response phenomenon described below.
Liquid crystal is generally considered to respond to the effective values (rms) of the driving waveform. Assuming that an effective voltage of V.sub.on (rms) is applied to a selected electrode and an effective voltage of V.sub.off (rms) is applied to an unselected electrode, a driving margin (V.sub.on (rms) / V.sub.off (rms)) takes the maximum value: ##EQU1## based on the voltage averaging method. In the above equation, N represents the number of scanning lines, and 1/N represents the duty ratio. Usually V.sub.off is set equal to a threshold voltage V.sub.th of the LC.
A liquid crystal panel having very quick response tends to deviate from such an inherent response mode (i.e., responding to effective values(rms)) and instead responds to the driving waveform itself, so that the transmittance value fluctuates corresponding to each frame. This phenomenon is referred to as a "frame response phenomenon".
Because of the frame response phenomenon, the off-transmittance increases even if the V.sub.off (for the unselected pixels) is set equal to V.sub.th. In the selected pixels, the actual transmittance is reduced although the optimum effective voltage of V.sub.on (rms) is being applied. Thus, the conventional linearly sequential driving method, when applied to a high-speed STN LC panel, can remarkably deteriorate the display contrast thereof.
Therefore, in order to maintain the optical contrast in a high-speed and high-resolution STN LC panel, it is necessary to drive the LC so as to suppress the frame response phenomenon.
On the other hand, a driving method called a multiple scanning line simultaneous selection driving method (also referred to as "active driving") has been proposed, which generates scanning selection pulses from an orthogonal matrix. By the active driving method, a plurality of scanning lines are simultaneously selected during one frame period in order to control the frame response phenomenon, thereby supplying a number of small scanning selection pulses for one scanning electrode during each frame period. Thus, the active driving method utilizes the cumulative response effect of LC so as to reconcile rapid response and high contrast.
According to the active driving method, input image data is subjected to an orthogonal transform process using an orthogonal matrix, and a signal corresponding to the transformed data is supplied from the data electrode side. From the scanning electrode side, scanning voltage pulses are applied corresponding to the elements of column vectors of the orthogonal matrix used for the transform. An orthogonal inverse transform performed on the panel side for the input image data reproduces the input image.
Active driving methods can be generally classified into an active addressing method (hereinafter referred to as the "AA method") and a multiline selection method (hereinafter referred to as the "MLS method"), although both are based on the same principle. For detailed descriptions of the AA method, see T. J. Scheffer, et al., SID' 92, Digest, p.228; Japanese Laid-Open Patent Publication No. 5-100642; and the like. For detailed descriptions of the MLS method, see T. N. Ruchmongathan et al., Japan Display 92, Digest, pp.65-68, Japanese Laid-Open Patent Publication No. 5-46127, and the like.
FIGS. 1A to 1C show examples of respective orthogonal functions used for the AA method and two variants of the MLS method.
The AA method uses an orthogonal function such as the WALSH function shown in FIG. 1A. Positive or negative voltages (i.e., voltages corresponding to the elements 1! or -1! of the orthogonal matrix) are simultaneously applied to all of the scanning electrodes.
The MLS method, as in the conventional duty driving method, has unselected periods of scanning pulses. The elements 0! in the orthogonal matrices shown in FIGS. 1B and 1C correspond to the unselected periods. The MLS method has an advantage of using mathematical operations of a much smaller scale than the AA method because when an element of the matrix is 0!, the result of an orthogonal transform with given data (i.e., multiplication/addition) always becomes 0.
The MLS method is further classified into a dispersion MLS method (FIG. 1B) in which the selection pulses of the orthogonal function are dispersed through-out one frame period, and a non-dispersion MLS method (FIG. 1C) in which selection pulses of the orthogonal function are grouped into blocks. An example of the dispersion MLS method is a SAT (Sequency Addressing Technique) disclosed in Japanese Laid-Open Patent Publication No. 6-4049. An example of the non-dispersion MLS method is an IHAT (Improved Hybrid Addressing Technique) disclosed in T. N. Ruchmongathan et al., IDRC 1988 pp.80-85.
An intrablock dispersion MLS method (Japanese Patent Application No. 6-291848), in which the selection pulses are dispersed within each of a plurality of blocks into which one frame is divided, is classified as a non-dispersion MLS method in terms of its fundamental operation sequence, and therefore requires a smaller memory capacity than does the dispersion MLS method. However, hereinafter the intrablock dispersion MLS method and the dispersion MLS method will be collectively referred to as "the dispersion MLS method" because the intrablock dispersion MLS method is capable of reducing the number of simultaneously selected lines to that required by the dispersion MLS method.
In general, the dispersion MLS method is considered to provide the same effect, by using a smaller number of selected lines, as that of the non-dispersion MLS method. In fact, an experiment in which a VGA-class LC panel having a response speed of 150 ms was driven while being split into upper and lower halves so as to display an image at a frame frequency of 60 Hz showed that the dispersion MLS method only requires 7-15 lines to be simultaneously selected in order to attain the same contrast level as that attained by the AA method, which selects all of the 240 scanning lines. On the other hand, the non-dispersion MLS method required 60 or more lines to be simultaneously selected in order to attain the above-mentioned contrast level.
However, the memory capacity required for the orthogonal transform operation depends on the calculation order of the orthogonal transform operation, i.e., the specific orthogonal transform matrix chosen. Thus, the non-dispersion MLS method has an advantage in that it only requires a memory capacity corresponding to the number of selected lines, whereas the AA method and the dispersion MLS method fundamentally require a memory capacity for storing data corresponding to at least one entire frame. Therefore, neither the dispersion MLS method nor the non-dispersion MLS method is superior.
However, when contemplating a system which primarily aims to maintain a satisfactory contrast level, a smaller operation scale is desirable because it leads to lower power consumption. Therefore, the dispersion MLS method is considered the most practical among the various active driving methods for rapid STN LC panels.
As described above, among the various active driving methods for high-speed STN LC panels, the dispersion MLS method is considered to have the optimum balance between the contrast level and circuit scale.
However, the inventors discovered upon driving a high-speed STN LC panel by the dispersion MLS method, that the dispersion MLS method has problems unique to itself, e.g., degradation in display quality such as a double-image (ghost) phenomenon and display unevenness occurring in a horizontal zone as described below. These problems do not belong to the duty driving method.
The above-mentioned problems are ascribed to nothing but the operation principle of the dispersion MLS method, i.e., all the scanning lines are divided by the number of selected lines into a plurality of subgroups in such a manner that the scanning selection waveform is dispersed within each subgroup, as described below.
FIG. 2 shows an exemplary orthogonal function matrix used for the dispersion MLS method. In this case, there is a total of 8 scanning lines to be selected, two of which are simultaneously selected, and there are 8 data electrodes. In theory, the elements +1! and -1! of the orthogonal matrix correspond to scanning selection pulse potentials +V.sub.r and -V.sub.r, respectively, and the element 0! of the orthogonal matrix corresponds to a unselected potential V.sub.com (=0). Data shown in FIG. 3 is to be displayed by using the orthogonal function in FIG. 2. FIG. 4 shows the waveform of pulses to be applied to the scanning electrodes by a common driver IC on the scanning side for driving the LC.
In an actual LC panel module, the electrode resistance of the scanning electrodes e.g., those of ITO (Indium Tin Oxide), the ON resistance of the scanning-side driver IC, and the capacitance component of the LC itself form a low-pass filter, which cuts off the harmonics components contained in the steep rises and steep falls of the scanning pulses. As a result, the waveform of the voltage to be applied to the scanning electrodes is distorted (or blunted) as shown in FIG. 5 in actual operation.
Among the distortions of the waveform of scanning selection pulses, the distortion occurring at the foot of the falling edge of each pulse, which causes some degradation in the display quality, will be first described.
When such distortion occurs, the fall of the +V.sub.r pulse (or the rise of the -V.sub.r pulse) has some delay so that each scanning selection pulse is applied to the same scanning electrode for a period slightly longer than the intended period, as shown in FIG. 5.
With respect to the scanning electrodes S1 and S2, the first selection pulse in one frame is to be applied during a period t1. However, the above-mentioned distortion of the scanning selection pulse waveform is applied as a secondary selection pulse to the scanning electrodes S1 and S2 for a period of .DELTA.t in addition to the period t1. The period .DELTA.t exists within a period t2, during which a selection pulse is to be applied to the next scanning electrodes S3 and S4.
In other words, a data signal from the segment side is applied (as ON voltage) to portions of the LC corresponding to the scanning electrodes S1 and S2 during not only the intended period t1 but also the period .DELTA.t within the period t2, during which the selection pulse is to be applied to the scanning electrodes S3 and S4. As a result, the image data to be reproduced at positions corresponding to the scanning electrodes S3 and S4 are reproduced so as to be slightly visible at positions corresponding to the scanning electrodes S1 and S2, thus creating a ghost image. In summary, any waveform distortion occurring at the falling edge of a pulse allows an image which should be reproduced only under a selected number of scanning electrodes to be also reproduced under adjoining scanning electrodes, thereby resulting in a ghost or a faint image of the same pattern appearing at a position slightly shifted from the original image.
It may seem that the scanning electrodes S7 and S8 are free from the ghost phenomenon because they are located at the end of the 8 scanning electrodes, and also physically at an end of the LC panel. However, since the waveform distortion of the scanning selection pulse to be applied to the scanning electrodes S7 and S8 exists within the period during which the scanning electrodes S1 and S2 are selected, the image data to be reproduced at positions corresponding to the scanning electrodes S1 and S2 appear as a ghost at positions corresponding to the scanning electrodes S7 and S8. However, when the scanning goes back from the scanning electrodes S7 and S8 to the scanning electrodes S1 and S2, the function data (i.e., the orthogonal function) changes so that not just a simple ghost of the image to be reproduced at the scanning electrodes S1 and S2 but a reversed image (i.e., white portions appearing black and vice versa) of the ghost often appears at the scanning electrodes S7 and S8.
As a result, the display device data of FIG. 3 is likely to appear as in FIG. 6.
In the case of the duty driving method, scanning electrodes are sequentially selected one by one, so that the ghost of an image to be reproduced at the intended scanning electrodes, occurring due to waveform distortion at the falling edge of the scanning selection pulse, appears in principle at scanning electrodes next to the intended scanning electrodes, rather than at a position substantially away from the intended scanning electrodes as in the case of the active driving method. Moreover, the duty driving method selects a scanning electrode only once in every frame, so that any waveform distortion of a scanning selection pulse within one frame has a smaller influence than in the case of the active driving method, which selects a scanning electrode a plurality of times in every frame. Furthermore, the duty driving method is typically adopted for a low-speed panel, which has a thicker LC layer than that of a high-speed panel, that is, the capacitance component is smaller than in the case of a high-speed panel. Therefore, the influence of waveform distortion becomes even smaller. Thus, the double-image phenomenon of an original image being accompanied by a ghost image is not as prominent in the duty driving as in the active driving.
Next, the degradation in display quality due to waveform distortion occurring at the rising edge of a pulse will be described. The following description illustrates a case where the data signal is intended for displaying an all-white image.
When an orthogonal transform is performed for a normally-black LC panel by a binary digital system, white data corresponds to "1" (i.e., High) and black data corresponds to "0" (i.e., Low). Elements +1! and -1! correspond to "1" (i.e., High) and "0" (i.e., Low), respectively.
An orthogonal operation by this system is performed by taking an Exclusive OR of each column vector of the data and the function, and adding the results of the Exclusive ORs by an adder, the result of the addition defining a data signal corresponding to display data (i.e., a signal to be applied to the data electrodes). Accordingly, it is presumable that the operation result has a large dependence on the function when the data is all-white, i.e., all "1" (High).
Now a case will be contemplated where the orthogonal function matrix in FIG. 2 is used for an LC panel system composed of 8 scanning electrodes and 8 data electrodes (as in the above description of the double-image phenomenon). Herein, the display data is assumed to be all-white. The signal waveform on the data side of the circuitry in this case is constant irrespective of the data electrodes, as shown in FIG. 7. As seen from FIG. 7, the data signal waveform drastically varies only at a boundary between the period t4 and the period t5, at which the orthogonal function changes.
In the duty driving method and the MLS methods, unselected periods are predominant in the scanning signal waveform for every frame. Therefore, the change in the data signal on the segment side is induced to the common side, thereby appearing as an induction distortion in the waveform of the scanning signal.
In this exemplary case, the data signal changes only once in one frame, i.e., at the boundary between the periods t4 and t5 as shown in FIG. 8, and therefore does not cause induction distortion in any other periods in the frame. In other words, among scanning selection pulses, only the rise of the selection pulse applied to the scanning electrode S1 during the period t5 and the fall of the selection pulse applied to the scanning electrode S2 during the period t5 are influenced by the induction from the segment side (data electrodes).
Specifically, the selection pulse voltage for the scanning electrode S1 has a small amount of waveform distortion relative to the distortion of selection pulses for the scanning electrodes S3 to S8, whereas the selection pulse voltage for the scanning electrode S2 has a large amount of distortion relative to the waveform distortion of the selection pulses for the scanning electrodes S3 to S8. However, the scanning selection pulses for scanning electrodes other than the scanning electrodes S1 and S2 are not influenced by the induction from the segment side. For similar reasons, the selection pulse voltage level for the scanning electrodes S1 and S2 largely decreases at the beginning of the period t1 due to waveform distortion.
As a result, the waveform distortion occurring at the rising edge of the scanning selection pulses for the scanning electrodes S1 and S2 in the periods t1 and t5 is different (i.e., more or less drastic) from the waveform distortion occurring at the rising edge of the other scanning selection pulses. Therefore, the effective values of the applied voltages to the pixels (LC) corresponding to the scanning electrodes S1 and S2 become smaller than the effective values of the voltages applied to the pixels (LC) corresponding to other scanning electrodes.
Because of the difference between the effective voltages corresponding to the scanning electrodes S1 and S2 and the effective voltages corresponding to the scanning electrodes S3 to S8, the illuminance of a portion corresponding to the scanning electrodes S3 to S8 is lower than the illuminance of portions corresponding to the other scanning electrodes, thereby resulting in a horizontal zone (corresponding to the two scanning electrodes) of uneven or reduced illuminance. In summary, any difference between the waveform distortion at the rising edge of a scanning selection pulse corresponding to a point of change in the orthogonal function and the waveform distortion at other portions of the orthogonal function results in a horizontal zone (corresponding to the number of selected scanning electrodes) of unevenness in illuminance.
Although the effective values of the voltages applied to the pixels corresponding to the scanning electrodes S1 and S2 are different from each other in the above description, they become substantially equal in actual driving because of processes such as averaging the frequency of scanning selection pulses and rotation of the orthogonal function for cancelling the DC component.
Because of the above-mentioned difference in the waveform distortion at the rising edge of each scanning selection pulse and because of the waveform distortion at the falling edge of the scanning selection pulse, a zone of display unevenness as shown in FIG. 9B is observed when the image data shown in FIG. 9A is displayed on a display panel of 8.times.8 display pixels by using the orthogonal function of FIG. 2.
Thus, the dispersion MLS driving method has the above-mentioned problem of display unevenness due to the operation principle thereof, i.e., all the scanning lines are divided by the number of selected lines into a plurality of subgroups in such a manner that the scanning selection waveform is dispersed within each subgroup.